Demand for electric vehicles is increasing in recent years because of social requirements for lower fuel consumption and lower exhaust emissions.
In one of commercially available electric vehicles, as disclosed in the following patent document 1 for example, an internal combustion engine is coupled through a power dividing device (for example, planetary gear set and the like) to a first motor (electric motor/generator) provided primarily as a generator and a second motor provided primarily as a motor to drive wheels. The vehicle is driven in an engine operation mode (1), an EV operation mode (2) and a regenerative braking mode (3). In the engine operation mode (1), the vehicle is driven with only the drive power from the engine or both drive powers from the engine and the motor. In the EV operation mode (2), the vehicle is driven with only the drive power from the motor with the engine being stopped. In the regenerative braking mode (3), the kinetic energy generated at the deceleration or braking of the vehicle is converted into the electric energy, which is charged into a battery.
The drive system according to patent document 1, however, needs two systems, each of which includes a motor, an inverter and the like. The drive system therefore results in a large-sized, high-cost system. It is therefore not suited to a small- or compact-sized, low-cost vehicle, in which a system mounting space is limited.
In another electric vehicle, as disclosed in the following patent document 2, only one motor is directly coupled to the output shaft of an internal combustion engine to reduce costs. In this drive system, the vehicle is driven solely by the engine as engine-only operation or driven jointly by the engine and the motor as assist operation.
The drive system according patent document 2, however, cannot perform EV operation or regenerative braking efficiently, because the motor is directly coupled to the output shaft of the engine and hence influenced by the friction loss of the engine.
In another electric vehicle, as disclosed in the following patent document 3, a motor is coupled to the output shaft of an internal combustion engine through a clutch. In this system, the engine and the motor are engaged or disengaged by the clutch so that an EV operation or regenerative braking may be performed efficiently.
The drive system according the patent document 3, however, needs a large-sized clutch, which can receive the whole drive power from the engine, because the clutch is coupled to the output shaft of the engine. Further, because the engine, the clutch, the motor and the transmission unit are all coupled in series, the power transfer path becomes lengthy and is not suited to be mounted in small- or compact-sized vehicles.
In a still another electric vehicle, as disclosed in the following patent document 4, a planetary gear set is interposed between an internal combustion engine and a motor. The output shaft of the engine is directly coupled to a sun gear of the planetary gear set. The rotary shaft of the motor and the input shaft of a continuously variable transmission unit are coupled to a carrier of the planetary gear set. The drive power of the engine and the motor are transferred to wheels of the vehicle through the continuously variable transmission unit so that engine operation (engine-only operation or assist operation), an EV operation and regenerative braking may be performed.
The drive system according to the patent document 4, however, needs a large-sized continuously variable transmission unit, which can receive all the drive power from both the engine and the motor, because the drive power from the engine and the motor is transferred to the wheel side through the continuously variable transmission unit. As a result, it is not suited to be mounted in small- or compact-sized vehicles.    Patent document 1: U.S. Pat. No. 5,722,502 (JP 9-46821A)    Patent document 2: U.S. Pat. No. 5,862,497 (JP 9-280085A)    Patent document 3: JP 2003-200743A    Patent document 4: U.S. Pat. No. 6,302,227 (JP 2000-219055A)